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Karman Clouds

Island Cloud

This image was taken by a European Space Agency satellite last month, it shows the Juan Fernandez islands, 800 km off the Chilean coast in the Pacific Ocean. The two largest of these islands are 1650 and 922 m high, this is high enough to affect the cloud patterns making pretty patterns in the sky!

As the wind flows past the island the mountains cause drag – wind speed increases further above land as the land causes friction (shown in the picture below). The change in speed causes regions of the atmosphere to have sharp velocity gradients. Where there are large changes in the wind speed vorticity occurs, leading to the observed vortices as the wind passes the mountainous islands.

Wind_Speed_Increase_Height

This process, known as Karman vortices, depends upon the wind speed and island size, for smaller islands a larger wind speed is needed for the patterns to appear. It isn’t only on this large scale that Karman vortices are found though. They can also affect building so engineers need to be aware of the atmospheric turbulence they can cause when building skyscrapers and Karman vortices can also cause car aerials to vibrate at certain speeds.

experimentalvortex

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As well as the CTD work I’m involved in and the WAGES work with buoys and balloons there is one other science team on board. They are from the National Oceanography Centre Liverpool, linked with Southampton. They are here to deploy and recover bottom pressure recorders. These are dropped over the side of the ship and sink to the bottom. They sit on the bottom for a year or more gathering data. When we come along to collect them an acoustic signal is sent out to ‘wake up’ the bottom pressure recorder, allowing the bottom pressure recorder to ‘talk’ to the ship. Then a signal is sent to make the bottom pressure recorder release from the bottom, by leaving a heavy frame behind, and float up to the surface. The bottom pressure recorder is then recovered back on board. As the instrument has been on the sea bed for at least a year some of the local wildlife is often brought back up with it! As you can expect, being brought up to the surface from around 1000 m depth is not a good idea.
Here are some of the specimins we found:
 

As I’m not biologically inclined I won’t many any attempts to identify them!
 

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Yesterday we let Pudsey go for a ride on the WAGES spar buoy, which sits behind the boat measuring wave motion. He seemed to enjoy it and came back dry!

Then he wanted to go up with the WAGES balloon. This is designed to take photos of the sea surface to allow them to evaluate how much white capping (white horses) there is.

As we get closer to Antarctica there has been an increase in the number of penguins we are spotting. Most of these are Chinstraps, which are constantly yelling for their friends making them easy to spot. It seems odd seeing these birds so far from land in the middle of nowhere, often on their own, but they look happy!

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What I Do

I have told you that we ‘do CTD stations’ but not what that involves doing specifically. The CTD (conductivity-temperature-depth) itself is a ‘rosette’ of bottles which are filled by ‘firing’ them (click go on the computor which causes a magnet to release closing the bottle) to collect water samples at various depths. There is a selection of sensors monitoring not only conductivity (which is used to calculate salinity) and temperature, but also pressure, oxygen, fluorescence, transmissivity and altimeter depth (within the bottom 100 m). The data from all these is fed straight to the CTD operator’s computer, in this case me. The CTD is lowered over the side by a winch with a 10000 m cable; it drops at 60 m/min. On the way down (the downcast) no stops are made so that we can collect a continuous profile of the water properties. On the way back up stops are made to collect water in the bottles. The water samples are used to calibrate the salinity measured by the CTD with lab tests. Other tests can be carried out on the samples. 


My role is operating the CTD. Before we get to the station I set up a file for the data to be collected in and make sure the graphs will display the whole water column. When we arrive I turn on the pump (to pump water past the sensors) and start collecting the data. I fill in a log sheet, recording the time, date, depth and various other parameters. The winch man lowers the CTD to 10 m, where we wait for the pump to activate before bringing the CTD back up to the surface and lowering straight down to the bottom. The descent takes over an hour on the deeper stations. Towards the bottom I keep an eye on the altimeter – we want to stop the CTD about 10 m above the sea bed. 
I fire two of the bottles at the bottom, then ask to winch man to bring the CTD up to the next depth to be sampled. I decide the sampling depths by looking at the salinity profile taken on the downcast and choosing areas of low salinity gradient (why?). Five or six depths are sampled, including just below the surface, two bottles are fired at each in case one does not fire correctly or leaks when it comes out of the water.



A few hours after the CTD entered the water it comes back out. When the bottles are filled it can weigh half a tonne – I’m glad it isn’t my job to manoeuvre it back onto the deck! The crew move the CTD back into its home. I record the on-deck pressure then stop collecting the data. There are a few initial data processing steps I complete before transferring the data to our directory. 

Then I put on waterproofs, boots and a hard hat and go down to do the water sample. We transfer water from the bottles on the rosette into smaller bottle; that are put in a controlled temperature lab for salinity testing. We empty the remaining water out onto the deck – my favourite bit! Then we cock the bottles in preparation for the next station, this is a bit awkward as the bottles are about a metre high and the top and bottom are strongly sprung. We have a system where I hold the top and bottom whilst the other scientist reaches into the middle of the rosette to secure them in place.
Between the stations I start processing the data and producing the preliminary plots to make sure the data looks realistic – unlike the density section posted previously. The ocean does not align itself in such perfect layers. If I manage to work out what went wrong there I’ll post a proper density section!
It has also been pointed out that a Hercules is not a fighter jet, it is a transporter. I should have been able to figure this out myself from its size but fighter jet sounds more exciting.

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Wind Stops Play

The attentive among you may have noticed the sudden splurge of activity on this blog. I have been writing a blog at day whilst at sea, but I haven’t managed to upload them all. Today we have been unable to do ‘science’ due to the sea state, so I have been catching up! I tried taking photos of the waves but you can’t tell the size with one shot, whilst we are heave to (head to wind) there isn’t much spray.
Coming into the labs before breakfast this morning I was greeted by a desk of woolen ear warmers, the overnight team had completed one task – learning to crochet! At their first station last night the rough sea had prevented the deployment of the CTD, so we had stayed put til light. At dawn the captain decided it was still to rough to put something overboard that takes a few hours to get back in. Coming up to 5 pm we are still waiting for the waves to calm. The WAGES team are using our delay productively with their motion buoy out collecting data all day.
Between watching the waves I have been processing the data we had collected so far. I have made some pretty plots:
These show the temperature, salinity and density of the water. We started the section on the left hand side – you can see the depth increasing as we get further into Drake’s Passage. From where we are (on the right hand side) the depth will stay about the same until we get close to Elephant Island on the other side. On the temperature plot (the left hand side) you can already see that the surface waters are getting colder as we head South. When there are more plots the interpretation will get more exciting with some different features (hopefully!).
This shows a T-S (temperature salinity) plot. The black contour indicate isopycnals – different density levels. The surface waters are at the top left side where temperature is highest and salinity lowest – causing least dense waters. The denser, colder and saltier waters are located towards the bottom right. The shape of the graph shows changing water bodies as you go down the water column.

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Chucking out an Argo

On my watch today we completed another couple of CTD (conductivity-temperature-density) stations – as I finished we were at station 9 (out of 30).  IAt the first station of the day we also deployed an ‘Argo float’.
Argo floats are drifting probes used to collect data about the properties of the ocean. There are thousands of Argo floats in the ocean, profiling the upper 2000 m of the water column. Every 10 days the floats surface and send data to scientists via satellites. Initially the floats collected only temperature and salinity data, though many extra sensors have been added to newer floats recording biological and chemical properties as well.
The project is a large collaboration with at least 31 countries involved. The float we deployed was funded by the UK Met Office. We have 5 more to deploy at various locations along our cruise.
To deploy it the float was activated on the deck, then lowered into the water and let go (lowered carefully, not just chucked). It didn’t seem to do anything whilst we watched it drift out of reach; a few giant petrels gave it a peck. Hopefully it sank down to its neutral buoyancy level and started collecting data.
The floats are carefully lowered into the ocean from ships all over the world – not just research ships, whatever boat is going to the region of the ocean that is required. We have six to deploy at various locations during my trip, funded by the UK Met Office. The programme is international with floats located globally and funded by many different countries.

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There are about 20 of us with British Antarctic Survey (BAS) going on the James Clark Ross (JCR) to Rothera. From there some fly back to the UK, some stay on the JCR (me included), some are staying in Rothera for weeks or months, a few are overwintering – spending the next 16 months on the frozen continent. After arriving in the Falkland Islands on Monday afternoon we had free time until
Thursday morning, when we join the JCR to prepare to head South, so we went penguin bothering!

Although I just have a cheap compact camera lots of people have proper cameras; I will add some of their photos when I get them but see frictionvelocity for more in the meantime.

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